The present invention relates to x-ray imaging, and more particularly, to x-ray imaging techniques that can distinguish different elemental substances within the object being imaged.
In x-ray transmission imaging, illumination is projected through an object and the imaging signal results from a subtractive process, i.e. what is imaged is the far field of the illuminations minus any signal that was absorbed, reflected or scattered. In many industries x-ray inspection is being used routinely for inspection of products in the manufacturing environments. Many of these applications require that a particular material be examined in the inspection process in spite of the presence of other materials that also absorb x-rays. For example, surface mounted integrated circuits are examined during manufacture to determine the distribution of solder on the assembly, which in turn, is related to the reliability of the assembly. In such systems, Cu traces, Si chips, and Fe in transformers all conspire to obscure, or xe2x80x98shadowxe2x80x99 the solder joints. Similar problems exist today in the use of x-ray imaging in the food processing industry for foreign object identification, the molding and casting industry for real-time mold-fill visualization, the transportation industry for contraband detection, etc. Hence, techniques that provide some specificity to distinguish between the various overlaid features are needed. In the following discussion, xe2x80x9cspecificityxe2x80x9d is defined as the ability to enhance the contrast of one material with respect to the contrast of an interfering material.
One example where specificity has been accomplished is in the medical use of dual energy imaging to measure bone density and/or body fat. By exploiting the fact that bone absorbs x-rays to higher energies, two images are taken using different accelerating voltages in the x-ray tube. Simple algebra is used to combine these images in a manner that accentuates the less or more absorbing x-rays and provide a xe2x80x98bone imagexe2x80x99 or a xe2x80x98fat imagexe2x80x99. However, this technique requires that two images with exact registration must be taken; hence, any motion of the object (patient) shows up as an inaccuracy. Since it takes several seconds to reset the tube energy, these registration artifacts are difficult to eliminate. In principle, two x-ray sources can be used to take the images, but this increases the system cost and requires periodic calibration to register the two sources.
In principle, the desired specificity can be provided by using a single energy source with a detector that has the ability to detect the energies of the x-rays that reach the detector. For example, U.S. Pat. No. 5,943,388 describes a discrete detector for use with a broad energy range polychromatic illumination source. In this approach, active pixels are created by discrete detectors coupled to a multi-channel analyzer that detects each x-ray photon arriving at each pixel. The photons are counted in bins corresponding to a pre-defined set of energy windows. Each energy window corresponds to a different image. By correctly choosing energy windows that are specifically affected by particular materials, significant improvements in specificity can be achieved. Unfortunately, the cost of the detectors for such a scheme significantly increases the cost of the imaging systems.
All of the previously discussed approaches rely on either imaging using two separately produced spectra with differing energy distributions, which requires the complexity of multiple sources or the delays of energy varying in real-time or energy discrimination in the pixelated detector, with the attendant electronic complexity and cost.
Additionally, there are a number of rastering schemes (similar to the operation of a scanning electron microscope) for building up a material-specific image using a well-defined x-ray spot and a single detector in which the image is reproduced pixel by pixel. Unfortunately, the throughput of such schemes is severely limited by two factors. First, it is difficult to focus x-rays (particularly at industrially relevant energies). A small spot must generally be created by collimation, which substantially reduces the flux of x-rays available for imaging. While this problem can be remedied by using a synchrotron to generate a collimated beam of x-rays, such accelerators are too expensive for routine imaging on production lines and the like. This renders these schemes only useful for failure analysis and scientific studies, as opposed to real-time quality control testing. Second, even if sufficient flux is obtained, the object must be moved for each pixel, requiring milliseconds per pixel to move and settle. Hence, these schemes are impractical if large images are needed.
Broadly, it is the object of the present invention to provide an improved apparatus and method for generating x-ray images having improved elemental specificity.
It is a further object of the present invention to provide an imaging instrument and method that is better suited to production line screening and the like.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an apparatus for imaging features of an object constructed from a material that includes a first element in the presence of features constructed from a material that includes a second element. The apparatus utilizes a filtered x-ray spectrum to image the object with the aid of an x-ray detector. The filtered x-ray spectrum is generated from a polychromatic x-ray source having a maximum energy that is determined by the absorption spectrum of the first element. The filter includes a filter element having a dominant absorption edge greater than or equal to the dominant absorption edge of the second element and less than the maximum energy. The filter removes x-rays that would otherwise be absorbed by the second element thereby improving the direct interpretation of contrast of images based on x-ray absorption of the first element.